A flux-gate magnetometer permits to detect a direct-current (dc) magnetic field with high sensitivity, however a carrier is about 100 kHz and a cutoff frequency thereof is no more than 1 kHz. A flux-gate magnetometer often uses a high-order low-pass filter, because of suppression of a switching surge included in an output waveform of a synchronous detector, thus increasing a phase lag in the vicinity of a cutoff frequency.
On the other hand, a search-coil magnetometer does not permit to detect a direct-current magnetic field, however it may be designed to have a relatively flat gain property relative to an alternate-current magnetic field from several Hz to tens of kHz. However, a signal-to-noise ratio (SN ratio) is inversely proportional to measurement frequency, and noise intensity is large on the side of low-frequency band and noise intensity is small on the side of high-frequency band.
Accordingly, the flux-gate magnetometer detects magnetic field of from the direct-current magnetic field to the alternate-current magnetic field of the low-frequency band, the search-coil magnetometer detects the alternate-current magnetic field of from a low-frequency band to a high-frequency band of about 20 kHz, and for the alternate-current magnetic field of an intermediate frequency band, an attribute fraction to an output may continuously be changed, together with the frequency, from the flux-gate magnetometer to the search-coil magnetometer. This makes it possible for the magnetic sensor provided with the flux-gate magnetometer and the search-coil magnetometer to detect the magnetic field of from the direct-current magnetic field to the alternate-current magnetic field of a high-frequency band of about 20 kHz with high sensitivity and at high resolution, without switching an operation mode from flux-gate magnetometer to the search-coil magnetometer
When an active magnetic shield is made based on a negative feedback control, a large loop gain is required to enhance a shield effect. However, a large phase lag of a sensor by which a magnetic field is monitored would cause a system to be prone to oscillate, resulting in difficulty in making a stable control system with high gain.
One of keys to the solution of this problem is to how to manufacture a magnetic sensor with less phase lag at a band ranging to a high-frequency band.
A natural way of thinking is to synthesize seamlessly at least two magnetic sensors having different frequency band to be detected, in frequency response characteristic.
As a contracting sensor, there exists a sensor in which a core (a magnetic core) of a flux-gate magnetometer is used as a core (a magnetic core) of a search-coil magnetometer, a direct-current magnetic field and an alternate-current field of a low-frequency band are detected by the flux-gate magnetometer and an alternate-current magnetic field of a high-frequency band is detected by the search-coil magnetometer. However, such a magnetic field sensor does not always (simultaneously) operate both of two operational modes by the flux-gate magnetometer and the search-coil magnetometer, but performs a switching operation between the two modes in a time shared manner, thus being inapplicable to an active magnetic shield in which a continuous control is required.
An attempt to place a flux-gate magnetometer and a search-coil magnetometer closely to each other to operate simultaneously the flux-gate magnetometer and the search-coil magnetometer would cause a problem that an excitation magnetic field (a modulation magnetic field) applied to a core (a magnetic core) of the flux-gate magnetometer may become an unnecessary input (a magnetic field interference) to the search-coil magnetometer.
For example, a conventional magnetic field measurement apparatus for a railroad vehicle is provided with a combined magnetic field sensor, which is made by combining integrally a first three-axis magnetic sensor for measurement of a magnetic field of ultralow-frequency response characteristic of a leakage magnetic field, in which three magnetic sensing units are mutually orthogonal, and a second three-axis magnetic sensor for measurement of a magnetic field of high-frequency response characteristic of the leakage magnetic field, in which three magnetic sensing units are mutually orthogonal with each other (see for example Patent Document 1).
Concerning a conventional broadband magnetic field compensation system, a magnetic field measurement apparatus is provided with the first sensor and the second sensor. The first sensor is active to a magnetic field within the first frequency range, and this frequency range includes a lower frequency than the frequency range within which the second sensor is active to a magnetic field (see for example Patent Document 2).